Turbochargers are well known and widely used with internal combustion engines. Generally, turbochargers supply more charge air for the combustion process than can otherwise be induced through natural aspiration. This increased air supply allows more fuel to be burned, thereby increasing power and torque obtainable from an engine having a given displacement. Additional benefits include the possibility of using lower displacement, lighter engines with corresponding lower total vehicle weight to reduce fuel consumption, and use of available production engines to achieve improved performance characteristics. Some turbocharger applications include the incorporation of an intercooler for removing heat (both an ambient heat component and heat generated during charge air compression) from the charge air before it enters the engine, thereby providing an even more dense air charge to be delivered to the engine cylinders. Intercooled turbocharging applied to diesel engines has been known, in some applications, to double the power output of a given engine displacement, in comparison with naturally aspirated diesel engines of the same engine displacement.
Additional advantages of turbocharging include improvements in thernal efficiency through the use of some energy of the exhaust gas stream that would otherwise be lost to the environment, and the maintenance of sea level power ratings up to high altitudes.
At medium to high engine speeds, there is an abundance of energy in the engine exhaust gas stream and, over this operating speed range, the turbocharger is capable of supplying the engine cylinders with all the air needed for efficient combustion and maximum power and torque output for a given engine construction. In certain applications, however, an exhaust stream waste gate is needed to bleed off excess energy in the engine exhaust stream before it enters the turbocharger turbine to prevent the engine from being overcharged. Typically, the waste gate is set to open at a pressure, above which undesirable predetonation or an unacceptably high internal engine cylinder pressure may be generated.
At low engine speeds, such as idle speed, however, there is disproportionately less energy in the exhaust stream as may be found at higher engine speeds, and this energy deficiency prevents the turbocharger from providing a significant level of boost in the engine intake air system. As a result, when the throttle is opened for the purpose of accelerating the engine from low speeds, such as idle speed, there is a significant time lag, i.e., turbo lag, and corresponding performance delay, before the exhaust gas energy level rises sufficiently to accelerate the turbocharger rotor and provide the compression of intake air needed for improved engine performance. The performance effect of this turbo lag may be more pronounced in smaller output engines which have a relatively small amount of power and torque available before the turbocharger comes up to speed and provides the desired compression.
Various efforts have been made to address the problem of turbo lag, including a reduction in the inertia of the turbocharger rotor. In spite of evolutionary design changes for minimizing the inertia of the turbocharger rotor, however, the turbo lag period is still present to a significant degree, especially in turbochargers for use with highly rated engines intended for powering a variety of on-highway and off-highway equipment.
Furthermore, to reduce exhaust smoke and emissions during acceleration periods when an optimal fuel burn is more difficult to achieve and maintain as compared with steady-speed operation, commercial engines employ devices in the fuel system to limit the fuel delivered to the engine cylinders until a sufficiendy high boost level can be provided by the turbocharger. These devices reduce excessive smoking, but the limited fuel delivery rate causes a sluggishness in the response of the engine to speed and load changes.
The turbo lag period can be mitigated and, in many instances, virtually eliminated by using an external power source to assist the turbocharger in responding to engine speed and load increases. One such method is to use an external electric power supply, such as the electrical energy stored in batteries, to power an electric motor that has been integrated into the mechanical design of a turbocharger. By providing the motor components within the turbocharger housing, the turbocharger bearings can also serve as motor bearings.
Providing motor components within a turbocharger assembly presents, however, a number of problems. Such motor components include permanent magnets to provide an electric motor rotor and wire windings to provide an electric motor stator, and the permanent magnets and stator windings must be in sufficient proximity to permit a relatively efficient conversion of electric energy applied to the stator windings into rotational energy imparted to the turbocharger rotor by the permanent magnets. The attachment of permanent magnets to the shaft exposes the permanent magnets to heat which is conducted down the shaft from the exhaust gas turbine wheel, and the exposure of the permanent magnets to such heat and their resulting temperatures may deleteriously affect the permeability and magnet field strength of the rotor magnets and result in insufficient and ineffective operation of the electric motor. In addition, the permanent magnets are exposed, in their rotation, to significant centrifugal forces since the turbocharger shaft can rotate at speeds up to 100,000 rpm and higher. The addition of stator windings within a turbocharger assembly also presents problems because the high temperatures that are reached in the turbocharger assembly can adversely affect the electrical insulation of the stator windings leading to possible failure.
A turbocharger assembly, including an integral assisting motor is disclosed in our prior U.S. patent application Ser. No. 08/680,671, filed Jul. 16, 1996, and U.S. Ser. No. 08/731,142, filed Oct. 15, 1996, which have addressed these problems and others.
Other patents disclosing turbocharger-electrical machine combinations include U.S. Pat. Nos. 5,406,797; 5,038,566; 4,958,708, 4,958,497; 4,901,530; 4,894,991; 4,882,905; 4,878,317, and 4,850,193.